This application claims the priority of German patent document 197 07 814.1, filed Feb. 27, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell energy generating system.
Full cell systems are used, for example, to provide electric energy in electric vehicles. In one known system in which hot-vapor reforming of methanol is performed in a reformate generating device, methanol and water can be stored in the vehicle in liquid form. The reforming reaction is adjusted such that the generated gas mixture (reformate) is high in hydrogen.
Carbon monoxide contained in the reformate is undesirable because it poisons the fuel cells. It is therefore normally removed in one or several gas purification stages connected downstream of the reforming reactor, for example, by a partial CO-oxidation or a selective separation of the carbon monoxide or of the hydrogen. A carbon monoxide sensor is used, for example, as a reformate-quality-indicating sensor, by means of which the carbon monoxide content in the reformate is monitored in order to carry out certain system control measures as a function thereof.
British Published Patent Application GB 2 250 130 A describes a fuel cell energy generating system of the initially mentioned type, in which the carbon monoxide content of the reformate is measured by a CO-sensor arranged on the input side of the CO-oxidation stage. The information thus obtained is used to adjust the amount of oxygen introduced into the CO-oxidation stage to a stoichiometric measurement in order always to ensure a complete CO-oxidation.
European Published Patent Application EP 0 710 996 A1 describes another fuel cell energy generating system of the initially mentioned type. In different variants, either only one CO-sensor is arranged on the inlet or outlet side of the fuel cell stack or two sensors of different sensitivities are arranged serially on the inlet side of the fuel cell stack. In each case, the respective output signal of the CO-sensor or CO-sensors is fed to a control unit which controls the operation of certain units of the reformate generating device as a function thereof; specifically, a reforming reactor, a CO-converter and a unit for carrying out a partial oxidation reaction. In both that document and European Published Patent Application EP 0 710 835 A2 (which discloses a largely similar system), the systems are controlled by controlling the air quantity introduced into the partial oxidation unit, by increasing the reaction temperature therein and/or in the reforming reactor, or by controlling the reaction temperature in the CO-converter. In the latter document, different constructions of CO-sensors are described; in particular, those which have the structure and method of operation of a single fuel cell, and to which a constant electric consuming device is connected or to which an external potentiostat circuit with a reference electrode is assigned.
In the above-mentioned conventional systems, when an excessive CO-content in the reformate is detected by the respective CO-sensor, even if the reformate generating is completely switched-off, residual reformate with a high CO-content will still reach the fuel cell arrangement, which can reduce the fuel cell activity.
One object of the invention is to provide a fuel cell energy generating system of the type described above, which reacts very rapidly to disturbances in the provision of reformate, and particularly prevents such disturbances from causing damage to the fuel cell arrangement.
These and other objects and advantages are achieved by the fuel cell system according to the invention, in a first embodiment of which a control unit controls a valve that releases or interrupts the flow of reformate gas from the reformate generating device into the fuel cell arrangement, as a function of the reformate quality detected by the sensor. Such interruption may be achieved, for example, by discharging the reformate via a branch line in the form of a bypass around the fuel cell arrangement. The flow of reformate into the fuel cell arrangement, which is released in the disturbance-free system operation, can therefore be interrupted immediately when an insufficient reformate quality is detected (for example, an excessive CO-content in the reformate), by a corresponding control of the valve. In this manner, the penetration of such harmful reformate constituents into the fuel cell arrangement is interrupted immediately. Although the reformate generating device continues to deliver reformate which has an insufficient quality for a certain time period even when switched off completely, such reformate does not arrive in the fuel cell arrangement; rather, it is discharged in a different manner. A poisoning of the fuel cell arrangement, for example, by an excessive amount of carbon monoxide, can therefore be avoided in this fashion.
As a second alternative, which may be provided instead of or in addition to the former alternative, the system has a supply air control valve in the oxygen feeding pipe for the fuel cell arrangement. This valve is controlled by the control unit to regulate the air or oxygen supply to the fuel cell arrangement as a function of the reformate quality detected by the sensor. By a corresponding change of the oxygen supply to the fuel cell arrangement, temporarily higher carbon monoxide concentrations can be permitted in the reformate gas flow without damaging the fuel cells.
In a further embodiment of the invention, one reformate-quality-indicating sensor is provided on the inlet side and one on the outlet side of a reformate purification unit connected downstream of the reforming reactor. In this manner, the reformate quality is monitored at different points of the reformate system, and thus in a very reliable manner. The closer a sensor is positioned in front of the fuel cell arrangement, the more completely it monitors the operation of the whole reformate generating device. On the other hand, a sensor situated farther away from the fuel cell arrangement, provides a long early warning time if an insufficient reformate quality occurs there because of an operational disturbance in the part of the system situated upstream.
According to another embodiment of the invention, the respective sensor has the structure of one of a plurality of fuel cells which form the fuel cell arrangement. A sensor constructed in this manner has the same susceptibility to deteriorations of the reformate quality, particularly with respect to the CO-content in the reformate, as the actual fuel cell arrangement. However, it is smaller and can be monitored in a selective and more precise manner so that it can more rapidly detect changes in the reformate than is possible on the actual fuel cell arrangement.
In still another embodiment, a reformate-quality-indicating sensor is arranged upstream of the inlet side of the reforming reactor, and senses the composition of the gas mixture to be reformed. This sensor can already detect a faulty composition of the gas mixture to be reformed so that the flow of reformate having an insufficient quality into the fuel cell arrangement can be prevented very early.
Finally, in still another embodiment, an oxygen-containing gas flow, taken from an air or oxygen feeding line of the fuel cell arrangement, is fed on the cathode side to a reformate-quality-indicating sensor which has the structure of an individual fuel cell of the fuel cell arrangement. As a result, this sensor can simultaneously monitor the air supply or oxygen supply to the fuel cell arrangement.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.